The invention relates to an adsorbent precipitated on a carrier (support), a method of producing said adsorbent and a method of using said adsorbent.
It is known to use precipitates of hexacyanoferrate(II) and of hexacyanoferrate(III) to adsorb Cs+, in particular radiotoxic 137Cs+. The precipitates can be synthesized via a precipitation reaction of the corresponding starting compounds in an aqueous system. However, the precipitates come down in the form of finely crystalline or colloidal precipitates. Therefore, when the finely crystalline or colloidal precipitates are used in fixed-bed packing, high pressure drops ensue, which either reduces the through-flow rate of the Cs+-polluted water or limits the size of the adsorber columns used.
DE 40 21 046 A1 describes a Cs+-selective ion exchange material composed of diammoniumcopper hexacyanoferrate, which is deposited on a carrier consisting of a spherical porous anion exchanger based on polystyrene, wherein quaternary ammonium groups act as anion exchange groups. The quaternary ammonium groups bind the diammoniumcopper hexacyanoferrate deposited on the carrier. However, the swellability of the ion exchange material used as carrier has an adverse effect on the dimensional stability of an ion exchange material used as filter bed, since there is a volume expansion of the filter bed as a consequence of the swelling. As a result, mechanical stresses build up in a closed adsorptive apparatus, e.g., an adsorber column, and may damage the adsorptive apparatus. The abrasion resistance of the ion exchange carrier further decreases in the swollen state thereof and may result in carrier material being carried out of the adsorptive apparatus together with the Cs+ deposited thereon, which is undesirable in relation to the removal of radiotoxic 137Cs+ in particular. The carrier with its quaternary ammonium groups is a strong basic anion exchanger, so use of the carrier in an acidic medium gives rise to acid-base reactions which evolve heat which can reduce the Cs+ adsorption capacity of the adsorbent deposited on the carrier.
U.S. Pat. No. 4,448,711 describes impregnating a zeolite with metal ions capable of adsorbing sparingly water-soluble ferrocyanides in the pores of a zeolite through ion exchange. Following adsorption of the aforementioned metal ions in the pores of the zeolite, the zeolite is treated with an aqueous solution of soluble ferrocyanide, thereby forming a metal ferrocyanide of the aforementioned metal ions on the zeolite. The zeolite is subsequently subjected to an aging treatment. The aging treatment is carried out by heating the zeolite to 100° C. in water or air in order to promote the crystallization of the metal ferrocyanide compound. Alternatively, the zeolite is dipped into a highly concentrated aqueous solution of a neutral alkali metal salt at preferably 80-100° C. in order to remove unconverted metal ions and age the metal ferrocyanide compound in the pores of the zeolite. The metal ferrocyanide deposited on the zeolite carrier is costly and inconvenient to make because of the aging treatment needed. Moreover, not all zeolites, i.e., compounds of the type M2/2O.Al2O3.xSiO2.yH2O (M=mono- or polyvalent metal, z=valence, x=1.8 to 12, y=0 to about 8), are stable in both acidic and alkaline solution, narrowing the pH spectrum of Cs+-containing solutions which can be treated with the adsorbent deposited on the zeolite. Zeolites further tend to be produced in the form of a finely divided powder. When the zeolite powder is used in an adsorber column in the form of a fixed-bed packing, appreciable pressure drops ensue in the operation of the adsorber column. True, a granular zeolite could be produced from the pulverulent zeolite to reduce the pressure drops. Yet this would make the production of the zeolite-based carrier even more costly and inconvenient.